An area of network communications that has been evolving since the 1980's is the area of integrated services digital network (ISDN) systems. Since its inception, ISDN has provided a wide variety of services, including voice and data services with bit rates of up to 64 Kbps, integrated within a single network. For voice communications and many text and data applications, the 64 Kbps ISDN rate is sufficient. However, there are increasing demands for broadband communications with substantially higher bit rates such as, for example, high-speed data communications, video, and high resolution graphics communications. Thus, a second generation of ISDN, referred to as broadband ISDN (B-ISDN), has developed to support these latter types of communications while still providing the same advantages of the first generation ISDN.
The first generation I SDN utilizes a synchronous transfer mode wherein, for the duration of a connection, a synchronous channel with constant bit rate (CBR) is allocated to that connection. Although suitable for certain applications, synchronous transfer mode is generally unsuitable for integration of those latter service types which have bit rate requirements above 2 Mps. This has lead to the development of asynchronous transfer mode (ATM) as the preferred method for transferring information in a B-ISDN system. ATM is a method wherein a user's data is partitioned into fixed-length cells for transmission without a specified timing requirement, thereby enabling cell transfer over a synchronous or asynchronous network.
Voice, video, image and high-speed data communications require high-speed transmission capabilities to support associated high-bandwidth requirements; optical transmission systems are especially suitable for this purpose. Accordingly, B-ISDN systems typically employ synchronous optical network (SONET) technology as a transport system. The International Telecommunications Union Telecommunications Standardization Sector (ITU-T, formerly the CCITT) essentially describes a B-ISDN network as a system implemented with ATM and SONET technologies built on the concepts of the ISDN model.
FIG. 1 is a block diagram of an exemplary B-ISDN network. Network 100 includes a SONET physical carrier transport system 102 for transporting data between ATM switches 104, 106 and 108. ATM switches 104-108 provide a network interface for a user's customer premises equipment (CPE), such as a PBX or local computer. In the exemplary network of FIG. 1, ATM switch 104 couples CPEs 110, 112, and 114 with network 102, while ATM switches 106 and 108 connect CPEs 116, 118 and 120, respectively, to that network. The function and operation of ATM switches 104-108, also referred to as ATM nodes, are described further herein.
In contrast to the circuit switching orientation of the first generation ISDN, B-ISDN has a cell relay or packet-switching orientation. FIG. 2 illustrates a format of a cell-based transmission 200 generated by an ATM switch such as switch 104. Cell-based transmission 200 includes a plurality of cells 202, each of which has a size of 53 bytes, including a 5-byte header 204 and a 48-byte payload 206 of user data. Header 204 is generally used for achieving transmission over network 100 and, to that end, includes information pertaining to such functions as routing and traffic control. Segmentation of the data into cells results in an integrated approach to networking, providing flexibility for handling different types of information such as voice, data, image and video over a single transmission facility 102.
One of the many types of services which can be supplied by network 100 is a circuit emulation service, wherein user data flows through the network at a constant bit rate (CBR). This service typically uses ATM Adaptation Layer 1 (AAL1), which is specifically designed for CBR applications. Control information required for these operations is included in a control field 208. In the illustrative embodiment, control field 208 is a 1-byte field located in the 48-byte payload 206 dedicated to user data in each cell 202. Bits in successive control fields can optionally be used to convey a Residual Time Stamp (RTS), used in applications employing SRTS clock recovery. Depending on the implementation, source CPE 308 may provide AAL1 segmentation and reassembly, or those functions may be provided by source ATM switch 304.
Timing variations inevitably exist among the network components. High frequency variations, referred to as jitter, are typically reduced to manageable levels through the use of jitter filters in each node. Low frequency variations, referred to as wander, are typically dealt with in a network through the use of buffers located within the nodes of the network. Specifically, these buffers either repeat (i.e., underflow) or discard (i.e., overflow) blocks of data to compensate for differences in timing between the source and destination nodes. Underflow and overflow operations, generally referred to as slip, typically result in errors within the network. For example, in a voice circuit, slip may be detected by popping or clicking sounds, whereas in data transmissions, slip is manifested in the loss of data.
Various clock recovery techniques have been developed to maintain network synchronization of CBR services in ATM networks to avoid such loss of data and to ensure proper reception of CBR service traffic. These techniques include synchronous residual time stamp (SRTS), adaptive clock recovery (ACR) and synchronous network clocking (SNC) techniques.
In SRTS, timing information is typically carried through the network along with the data transmission. The destination node uses this timing information to recover the frequency of the source node clock which determines the frequency of a transmit clock that the destination node uses to transmit data to a destination CPE.
A drawback to the SRTS clock recovery method is that it is based upon the assumption that identical network reference clocks are provided to the source and destination nodes. Yet, this is often not the case because each portion of the network may be synchronized to a different reference clock creating multiple timing domains. Although these local clocks may both be referenced to a stratum 1 clock, the local clocks may experience a phase departure over time that continues to increase until a slip occurs. The use of different reference clocks is especially likely when the source and destination nodes are located in different countries.
Moreover, if a network element such as a digital cross connect fails, certain network nodes may lose their reference clock. These nodes must then utilize their local clocks, resulting in an increased loss of data due to the difference in phase and frequency between the node's local and reference clocks. The resulting phase departure further manifests itself as noted, as clicking and popping noises in voice transmissions and in the loss of data in image and video transmissions. Clearly, the SRTS clock recovery method is only as reliable as the consistency of the reference clocks utilized by the source and destination nodes.
The synchronous network clocking method has a similar problem, in that it mandates that the destination and source nodes utilize the network reference clock frequency as the sources for determining their respective local clocks. In properly operating networks this provides superior jitter and wander characteristics. However, unlike SRTS and ACR techniques, a network node implementing SNC cannot accept user-supplied clocking.
In ACR, the occupancy of a buffer receiving the incoming traffic is used to directly control the frequency of the destination node transmit clock. Since the adaptive clock recovery method does not rely on a common reference clock, it is typically used in systems wherein the distribution of a reference clock frequency is either undesirable or impracticable. The ACR technique maintains the data buffer near a target depth by varying the transmit clock rate about its nominal value. This achieves CBR service without requiring a network reference clock. However, adaptive clock recovery can create large amounts of wander as a side effect of adjusting the clock frequency, and is therefore primarily used in private, smaller networks rather than larger public networks.
Conventional network nodes utilize a particular clock recovery or network synchronization or clock recovery method based upon existing equipment and service priorities. For example, transmission between source and destination nodes which do not both have access to the same reference clock must use adaptive clock recovery technique. Further, a network node that cannot to be timed from a network reference clock cannot use the SNC technique. In addition, a network node implementing SRTS has an interoperability limitation of having to communicate with only nodes that implement SRTS generation techniques.
What is needed, therefore, is a means for ensuring the proper reception and transmission of data though a network that may be configured in accordance with a variety of user system timing requirements and to communicate with network nodes utilizing various types of clock recovery and network synchronization techniques.